In the application of a lithium ion secondary battery for electric automobiles and hybrid electric automobiles, further increasing the density of electric energy is required. At present, although LiFePO4 and the like are put to practical use as a highly safe positive electrode material, the average working electric potential thereof is as low as 3.4 V based on Li. In order to increase the energy density of the lithium ion secondary battery, a positive electrode active material with higher working electric potential is needed.
Recently, as a positive electrode material to meet the aforementioned requirement, Li(Fe, Mn)SO4F having a crystal structure of the tavorite type or triplite type is widely studied. As the average working electric potential of Li(Fe, Mn)SO4F of the tavorite type is 3.6 V (vs. Li/Li+), and the average working electric potential of Li(Fe, Mn)SO4F of the triplite type is 3.9 V (vs. Li/Li+), these are promising as next-generation secondary battery positive electrode materials with high energy density. However, because an electric potential change in electric charge and discharge of the positive electrode material of the tavorite type is small, the depth of charge is difficult to measure when used in combination with a negative electrode with similarly small electric potential change. And, the positive electrode material of the triplite type has a problem in that reduction in electric potential in a final stage of discharge is rapid, which leads to low output at the low depth of charge.